Place the oil in a large skillet over medium-high heat and then add the onion, ginger, and garlic. Cook for 4 minutes, until onions are soft and golden. Add the curry leaves, mustard seeds, sambar powder, asafoetida, red chile powder, and turmeric to the pan and cook for 2 minutes. Add the chicken and cook for 3 minutes, stirring constantly. Add the tomatoes, coconut milk, and water and cook until chicken is done, about 8 minutes. Serve the chicken with rice.
*Can be found at specialty Asian and Indian markets.